NMR study of spin polarized and superconducting phases of UTe₂ in high magnetic fields

The candidate triplet superconductor UTe₂ shows a unique interplay of magnetism and superconductivity (SC). Unlike other U-based ferromagnetic superconductors, it does not magnetically order. Superconductivity onsets at T_C ~ 2.0 K, coexisting with antiferromagnetic (AFM) fluctuations and a large internal magnetic field (H) distribution. At a high-H of H_m = 35 T, it shows a metamagnetic transition into a fully polarized state for H||b, and for a tilted H orientation (~20-40^o from H||b to H||c) above 40 T, an unusual reentrant superconducting state appears within the polarized phase, which suggests a close relationship between SC and magnetism. However, knowledge of local magnetic anisotropy and fluctuations is still lacking. To better understand these high-H polarized and superconducting states, we performed ¹²⁵Te nuclear magnetic resonance (NMR) experiments in the ¹²⁵Te-enriched single crystal of UTe₂ at field strengths near to, and greater than H_m. We will show our results of static and dynamic spin susceptibility measurements at high H and discuss the microscopic nature of reentrant superconducting phase. We will also analyze the possible relevance, as suggested recently, between the high-H states above H_m and the (low-field) high-pressure states above a critical pressure P_c = 1.5 GPa where magnetically ordered phases, which seem to be AFM, are stabilized.